There are a lot of metal baffles available, which could be inserted into that muffler. All baffles can be wrapped with fiberglass matting, to further reduce their sound output (use either
metal zip-ties, or fiberglass wood-burning stove-pipe rope to secure the matting in place). Search the Internet for these topics: muffler baffling, muffler fiberglass, etc.
With regards to back-pressure, and whether this affects the carburetion, that is a weird subject. Some folks claim re-jetting is required, some say it has no effect, and re-jetting is a bald lie. Don't know the truth, myself. Good luck with the research and self-educating from the Internet. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII